GWEN IFILL: Britain and the world marked the passing of former prime Minister Margaret Thatcher today. She was the first woman to lead any major Western power, and became a transformational figure at home and abroad.

Margaret Warner begins our coverage.

MARGARET WARNER: Britain’s longest serving prime minister of the 20th century died this morning after suffering a stroke.

Flags at Number 10 Downing Street and Buckingham Palace were lowered to half-staff, as an impromptu memorial appeared outside her London home, honoring the steely woman who had transformed her nation’s economy and politics and reasserted its voice in the world.

Current Prime Minister David Cameron, like Thatcher, a conservative, reflected on her legacy.

PRIME MINISTER DAVID CAMERON, Britain: As our first woman prime minister, Margaret Thatcher succeeded against all the odds. And the real thing about Margaret Thatcher is that she didn’t just lead our country; she saved our country. And I believe she will go down as the greatest British peacetime prime minister.

MARGARET WARNER: Thatcher came from humble beginnings, the daughter of a grocer in Central England. Yet she rose through Conservative Party ranks, winning a seat to Parliament in 1959 and later serving as minister of education.

Then, in 1979, after years of Labor Party domination, Thatcher led a tory resurgence that catapulted her to the office of prime minister, a post she held for more than 11 years.

FORMER PRIME MINISTER MARGARET THATCHER, Britain: Where there is discourse, may we bring harmony. Where there is error, may we bring truth. Where there is doubt, may we be faith. And where there is despair, may we bring hope.

MARGARET WARNER: The new prime minister brought a free market revolution to Britain, lowering taxes and privatizing state industries. In the early 1980s, she curbed the sweeping powers of Britain’s labor unions and triggered a year-long dispute with the national union of miners after she shuttered government-owned coal mines across the country.

MARGARET THATCHER: What we have got is an attempt to substitute the rule of the mob for the rule of law. And it must not succeed.

MARGARET WARNER: Britain’s economy rebounded from her tough medicine. And for her no-holds-barred leadership style, she was dubbed the Iron Lady. She clearly reveled in it.

MARGARET THATCHER: For those waiting with bated breath for that favorite media catchphrase the U-turn, I have only one thing to say. You turn if you want to.

The lady is not for turning.

MARGARET WARNER: But her unyielding policies aroused more than political hostility. In 1984, the Irish Republican Army bombed the Conservative Party conference in Brighton in a bid to assassinate her.

Still, years later, in a 1996 documentary, Thatcher maintained none of the criticism ever bothered her.

MARGARET THATCHER: Life isn’t fair. And there’s no point in getting too sensitive if you’re in politics. What you have got to be certain is that what you’re doing can be justified by principle, by argument, and to put it across. That’s the important thing.

MARGARET WARNER: She was just as hard-nosed in asserting Britain’s influence abroad. In 1982, she ordered British forces to reclaim the Falklands, after Argentina’s military junta invaded the islands. The war left about 650 Argentines and 255 Britons dead, but it earned Thatcher huge support at home.

In Washington, she found a kindred spirit in President Ronald Reagan, sharing his harder line toward the Soviet Union in the climactic final years of the Cold War. Yet when Thatcher met with incoming Soviet leader Mikhail Gorbachev in late 1984, she famously declared that we can do business with him.

Five years later, she was in power when the Berlin Wall came down. And in 1990, when Iraq invaded Kuwait, Thatcher backed a tough response, urging President George H.W. Bush not to go wobbly on confronting Saddam Hussein.

But back home, Thatcher’s own grip on power was wobbling. After 11 years in office, her public support flagged amid inflation and renewed recession. And the Conservative Party voted her out.

MARGARET THATCHER: We’re leaving Downing Street for the last time after 11-and-a-half wonderful years.

MARGARET WARNER: Even after her fall from power, Thatcher often drew large crowds at campaign events, nearly upstaging her successor, John Major, at a Conservative Party conference in 1992.

That same year, she was named a baroness. And for much of the ’90s, she made lucrative lecture tours. Margaret Thatcher’s withdrawal from public eye began in 2002, when a series of small strokes prompted her to cut back on public appearances and speaking events. It was the first of many health problems, including a struggle with dementia that shadowed her later years.

For a time, Thatcher did continue to appear at select private events and state functions. And in the summer of 2004, she returned to the United States for the funeral of former President Reagan, though she paid her respects in a pre-recorded video.

MARGARET THATCHER: We have lost a great president, a great American, and a great man. And I have lost a dear friend.

MARGARET WARNER: In 2005, Thatcher was well enough to attend her 80th birthday celebration at a London hotel and, in 2007, the unveiling of her statue in the Houses of Parliament.

In 2010, she made one of her last visits to 10 Downing Street at the invitation of Prime Minister David Cameron. After that, as depicted in the 2011 movie “The Iron Lady,” her descent into dementia kept her largely shut in.

Today, Queen Elizabeth authorized a ceremonial funeral with military honors for the former prime minister at Saint Paul’s Cathedral in London. Margaret Thatcher was 87 years old.

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